Access Control and Operating System Security

1. Access Control and Operating System Security John Mitchell

2. Access Control Matrix, ACL, Capabilities Multi-level security (MLS) OS Policies Multics Ring structure Unix File system, Setuid Windows File system, Tokens, EFS SE Linux Role-based Domain type enforcement Secure OS Methods for resisting stronger attacks Assurance Orange Book, TCSEC Common Criteria Windows 2000 certification Some Limitations Information flow Covert channels Outline

3. Access control • Common Assumption • System knows who the user is • User has entered a name and password, or other info • Access requests pass through gatekeeper • Global property; OS must be designed so that this is true ? Reference monitor Resource User process Decide whether user can apply operation to resource

4. Access control matrix [Lampson]

5. Two implementation concepts • Access control list (ACL) • Store column of matrix with the resource • Capability • Allow user to hold a “ticket” for each resource • Roughly: store row of matrix with the user Access control lists are widely used, often with groups Some aspects of capability concept are used in Kerberos, …

6. Capabilities • Operating system concept • “… of the future and always will be …” • Examples • Dennis and van Horn, MIT PDP-1 Timesharing • Hydra, StarOS, Intel iAPX 432, Amoeba, Eros, … • Reference • Henry Levy, Capability-based Computer Systems http://www.cs.washington.edu/homes/levy/capabook/

7. Roles (also called Groups) • Role = set of users • Administrator, PowerUser, User, Guest • Assign permissions to roles; each user gets permission • Role hierarchy • Partial order of roles • Each role gets permissions of roles below • List only new permissions given to each role Administrator PowerUser User Guest

8. Groups for resources, rights • Permission = right, resource • Group related resources • Hierarchy for rights or resources • If user has right r, and r>s, then user has right s • If user has read access to directory, user has read access to every file in directory • Big problem in access control • Complex mechanisms require complex input • Difficult to configure and maintain • Roles, other organizing ideas try to simplify problem

9. Multi-level Security Concepts • Military security policy • Classification involves sensitivity levels, compartments • Do not let classified information leak to unclassified files • Group individuals and resources • Use some form of hierarchy to organize policy • Other concepts • Separation of duty • Chinese Wall Policy

10. Satellite data Afghanistan Middle East Israel Military security policy • Sensitivity levels • Compartments Top Secret Secret Confidential Restricted Unclassified

11. Military security policy • Classification of personnel and data • Class = rank, compartment • Dominance relation • D1 D2 iff rank1  rank2 and compartment1  compartment2 • Example: Restricted, Israel  Secret, Middle East • Applies to • Subjects – users or processes • Objects – documents or resources

12. Product specifications Discontinued In production OEM Commercial version Internal Proprietary Public

13. Bell-LaPadula Confidentiality Model • When is it OK to release information? • Two Properties (with silly names) • Simple security property • A subject S may read object O only if C(O)  C(S) • *-Property • A subject S with read access to O may write object P only if C(O)  C(P) • In words, • You may only readbelow your classification and only writeabove your classification

14. Read above, write below Proprietary S Public Picture: Confidentiality Read below, write above Proprietary S Public

15. Biba Integrity Model • Rules that preserve integrity of information • Two Properties (with silly names) • Simple integrity property • A subject S may write object O only if C(S) C(O) (Only trust S to modify O if S has higher rank …) • *-Property • A subject S with read access to O may write object P only if C(O)  C(P) (Only move info from O to P if O is more trusted than P) • In words, • You may only writebelow your classification and only readabove your classification

16. Read below, write above Proprietary S Public Picture: Integrity Read above, write below Proprietary S Public

17. Problem: Models are contradictory • Bell-LaPadula Confidentiality • Read down, write up • Biba Integrity • Read up, write down • Want both confidentiality and integrity • Only way to satisfy both models is only allow read and write at same classification In reality: Bell-LaPadula used more than Biba model Example: Common Criteria

18. Other policy concepts • Separation of duty • If amount is over $10,000, check is only valid if signed by two authorized people • Two people must be different • Policy involves role membership and  • Chinese Wall Policy • Lawyers L1, L2 in Firm F are experts in banking • If bank B1 sues bank B2, • L1 and L2 can each work for either B1 or B2 • No lawyer can work for opposite sides in any case • Permission depends on use of other permissions

19. Example OS Mechanisms • Multics • Unix • Windows • SE Linux (briefly)

20. Multics • Operating System • Designed 1964-1967 • MIT Project MAC, Bell Labs, GE • At peak, ~100 Multics sites • Last system, Canadian Department of Defense, Nova Scotia, shut down October, 2000 • Extensive Security Mechanisms • Influenced many subsequent systems • http://www.multicians.org/security.html

21. Multics time period • Timesharing was new concept • Serve Boston area with one 386-based PC

22. Multics Innovations • Segmented, Virtual memory • Hardware translates virtual address to real address • High-level language implementation • Written in PL/1, only small part in assembly lang • Shared memory multiprocessor • Multiple CPUs share same physical memory • Relational database • Multics Relational Data Store (MRDS) in 1978 • Security • Designed to be secure from the beginning • First B2 security rating (1980s), only one for years

23. Multics Access Model • Ring structure • A ring is a domain in which a process executes • Numbered 0, 1, 2, … ; Kernel is ring 0 • Graduated privileges • Processes at ring i have privileges of every ring j > i • Segments • Each data area or procedure is called a segment • Segment protection b1, b2, b3 with b1  b2  b3 • Process/data can be accessed from rings b1 … b2 • A process from rings b2 … b3 can only call segment at restricted entry points

24. Unix file security • Each file has owner and group • Permissions set by owner • Read, write, execute • Owner, group, other • Represented by vector of four octal values • Only owner, root can change permissions • This privilege cannot be delegated or shared • Setid bits – Discuss in a few slides setid - rwx rwx rwx ownr grp othr

25. Question • Owner can have fewer privileges than other • What happens? • User gets access? • User does not? • Prioritized resolution of differences if user = owner then owner permission else if user in group then group permission else other permission

26. Effective user id (EUID) • Each process has three Ids (+ more under Linux) • Real user ID (RUID) • same as the user ID of parent (unless changed) • used to determine which user started the process • Effective user ID (EUID) • from set user ID bit on the file being executed, or sys call • determines the permissions for process • file access and port binding • Saved user ID (SUID) • So previous EUID can be restored • Real group ID, effective group ID, used similarly

27. Process Operations and IDs • Root • ID=0 for superuser root; can access any file • Fork and Exec • Inherit three IDs, except exec of file with setuid bit • Setuid system calls • seteuid(newid) can set EUID to • Real ID or saved ID, regardless of current EUID • Any ID, if EUID=0 • Details are actually more complicated • Several different calls: setuid, seteuid, setreuid

28. Setid bits on executable Unix file • Three setid bits • Setuid – set EUID of process to ID of file owner • Setgid – set EGID of process to GID of file • Sticky • Off: if user has write permission on directory, can rename or remove files, even if not owner • On: only file owner, directory owner, and root can rename or remove file in the directory

29. Example Owner 18 RUID 25 SetUID …; …; exec( ); program Owner 18 …; …; i=getruid() setuid(i); …; …; -rw-r--r-- RUID 25 file read/write EUID 18 Owner 25 RUID 25 -rw-r--r-- read/write EUID 25 file

30. Compare to stack inspection • Careful with Setuid ! • Can do anything that owner of file is allowed to do • Be sure not to • Take action for untrusted user • Return secret data to untrusted user A 1 B 1 C 1 Note: anything possible if root; no middle ground between user and root

31. Setuid programming • We talked about this before … • Be Careful! • Root can do anything; don’ t get tricked • Principle of least privilege – change EUID when root privileges no longer needed • Setuid scripts • This is a bad idea • Historically, race conditions • Begin executing setuid program; change contents of program before it loads and is executed

32. Unix summary • We’re all very used to this … • So probably seems pretty good • We overlook ways it might be better • Good things • Some protection from most users • Flexible enough to make things possible • Main bad thing • Too tempting to use root privileges • No way to assume some root privileges without all root privileges

33. Access control in Windows (NTFS) • Basic functionality similar to Unix • Specify access for groups and users • Read, modify, change owner, delete • Some additional concepts • Tokens • Security attributes • Generally • More flexibility than Unix • Can define new permissions • Can give some but not all administrator privileges

34. Sample permission options • SID • Identity (replaces UID) • SID revision number • 48-bit authority value • variable number of Relative Identifiers (RIDs), for uniqueness • Users, groups, computers, domains, domain members all have SIDs

35. Permission Inheritance • Static permission inheritance (Win NT) • Initially, subfolders inherit permissions of folder • Folder, subfolder changed independently • Replace Permissions on Subdirectories command • Eliminates any differences in permissions • Dynamic permission inheritance (Win 2000) • Child inherits parent permission, remains linked • Parent changes are inherited, except explicit settings • Inherited and explicitly-set permissions may conflict • Resolution rules • Positive permissions are additive • Negative permission (deny access) takes priority

36. Tokens • Security Reference Monitor • uses tokens to identify the security context of a process or thread • Security context • privileges, accounts, and groups associated with the process or thread • Impersonation token • thread uses temporarily to adopt a different security context, usually of another user

37. Security Descriptor • Information associated with an object • who can perform what actions on the object • Several fields • Header • Descriptor revision number • Control flags, attributes of the descriptor • E.g., memory layout of the descriptor • SID of the object's owner • SID of the primary group of the object • Two attached optional lists: • Discretionary Access Control List (DACL) – users, groups, … • System Access Control List (SACL) – system logs, ..

38. Example access request User: Mark Access token Group1: Administrators Group2: Writers Access request: write Action: denied Revision Number Control flags Owner SID • User Mark requests write permission • Descriptor denies permission to group • Reference Monitor denies request Group SID DACL Pointer Security descriptor SACL Pointer Deny Writers Read, Write Allow Mark Read, Write

39. Impersonation Tokens (setuid?) • Process uses security attributes of another • Client passes impersonation token to server • Client specifies impersonation level of server • Anonymous • Token has no information about the client • Identification • server obtain the SIDs of client and client's privileges, but server cannot impersonate the client • Impersonation • server identify and impersonate the client • Delegation • lets server impersonate client on local, remote systems

40. Encrypted File Systems (EFS, CFS) • Store files in encrypted form • Key management: user’s key decrypts file • Useful protection if someone steals disk • Windows – EFS • User marks a file for encryption • Unique file encryption key is created • Key is encrypted, can be stored on smart card • Unix – CFS [Matt Blaze] • Transparent use • Local NFS server running on "loopback" interface • Key protected by passphrase

41. Q: Why use crypto file system? • General security questions • What properties are provided? • Against what form of attack? • Crypto file system • What properties? • Secrecy, integrity, authenticity, … ? • Against what kinds of attack? • Someone steals your laptop? • Someone steals your removable disk? • Someone has network access to shared file system? Depends on how file system configured and used

42. SELinux Security Policy Abstractions • Type enforcement • Each process has an associated domain • Each object has an associated type • Configuration files specify • How domains are allowed to access types • Allowable interactions and transitions between domains • Role-based access control • Each process has an associated role • Separate system and user processes • configuration files specify • Set of domains that may be entered by each role

43. Secure Operating Systems • Extra mechanisms for extra security • Follow design and implementation procedures • Review of design and implementation • Maintenance procedures Will discuss • Mechanisms associated with secure OS • Standards for certification • Mostly used by government, some commercial interest

44. Sample Features of Trusted OS • Mandatory access control • MAC not under user control, precedence over DAC • Object reuse protection • Write over old data when file space is allocated • Complete mediation • Prevent any access that circumvents monitor • Audit • See next slide • Intrusion detection • Anomaly detection • Learn normal activity, Report abnormal actions • Attack detection • Recognize patterns associated with known attacks

45. Audit • Log security-related events • Protect audit log • Write to write-once non-volatile medium • Audit logs can become huge • Manage size by following policy • Storage becomes more feasible • Analysis more feasible since entries more meaningful • Example policies • Audit only first, last access by process to a file • Do not record routine, expected events • E.g., starting one process always loads …

46. Trusted path • Spoofing • Fool user/process into thinking they are communicating with secure part of system • Intercept communication • Trusted path • Mechanisms to prevent spoofing • Special key sequence for passwd command intercepted by trusted kernel (e.g, ctrl-alt-delete) • Allow some actions only at boot time, before user processes loaded

47. Kernelized Design • Trusted Computing Base • Hardware and software for enforcing security rules • Reference monitor • Part of TCB • All system calls go through reference monitor for security checking • Most OS not designed this way User space User process Kernel space OS kernel TCB Reference monitor

48. SELinux • Security-enhanced Linux system (NSA) • Enforce separation of information based on confidentiality and integrity requirements • Mandatory access control incorporated into the major subsystems of the kernel • Limit tampering and bypassing of application security mechanisms • Confine damage caused by malicious applications http://www.nsa.gov/selinux/

49. Why Linux? • Open source • Already subject to public review • This by itself does not guarantee security … • NSA can review source, modify and extend • Hope to encourage additional operating system security research • Released under the same terms and conditions as the original sources. • includes documentation and source code

50. Rainbow Series DoD Trusted Computer Sys Evaluation Criteria (Orange Book) Audit in Trusted Systems (Tan Book) Configuration Management in Trusted Systems (Amber Book) Trusted Distribution in Trusted Systems (Dark Lavender Book) Security Modeling in Trusted Systems (Aqua Book) Formal Verification Systems (Purple Book) Covert Channel Analysis of Trusted Systems (Light Pink Book) … many more http://www.radium.ncsc.mil/tpep/library/rainbow/index.html